This invention relates to a method for preparing linear polyimides from macrocyclic polyimide oligomers.
Linear polyimides and polyetherimides are well known commercially available polymers having advantageous properties including thermooxidative resistance, good mechanical strength, excellent electrical properties and good chemical resistance. They are normally prepared from at least one tetracarboxylic acid derivative, typically a dianhydride, and at least one diamine in a two-step method, the first step being reaction of the diamine with the tetracarboxylic acid or derivative thereof to form a polyamic acid and the second step being conversion of the polyamic acid to a polyimide with elimination of water.
Methods of this type are generally limited to the preparation of relatively thin polyimide films, since it is necessary to remove volatiles (e.g., water and the solvents employed) without causing formation of bubbles and voids. In addition, the polyimides are extremely difficult to process after formation because of their lack of solubility in common solvents, high glass transition temperatures and extremely high melt viscosities, typically in excess of one million poise at 300.degree. C. Thus, operations such as injection molding cannot be performed.
Macrocyclic polyimide oligomers are a genus of polyimide intermediates with which such operations may be employed. The preparation of the oligomers is disclosed in three copending applications, Ser. No. 08/341,815; Ser. No. 08/096,393, now U.S. Pat. No. 5,362,837; and Ser. No. 08/164,087, now U.S. Pat. No. 5,357,029. Their conversion to linear polyimides is disclosed in copending application Ser. No. 08/080,864, now U.S. Pat. No. 5,362,845. All of said applications are commonly owned herewith.
Said application Ser. No. 08/080,864 describes a method of conversion which requires the use of a primary amine as an initiator and a macrocyclic polyimide oligomer polymerization catalyst. The latter may be an alkaline earth or transition metal, a salt thereof or an alkali metal hydroxide or salt. As a result, the linear polyimide contains metal residues derived from the catalyst. The presence of such residues may have a deleterious effect on the properties of the polyimide, especially when it is to be used as a dielectric material. It would be desirable, therefore, to prepare linear polyimides from macrocyclic polyimide oligomers by a method which employs non-metal-containing catalysts.